tree-optimization/107160 - avoid reusing multiple accumulators

tree-optimization/107160 - avoid reusing multiple accumulators

Epilogue vectorization is not set up to re-use a vectorized
accumulator consisting of more than one vector.  For non-SLP
we always reduce to a single but for SLP that isn't happening.
In such case we currenlty miscompile the epilog so avoid this.

	PR tree-optimization/107160
	* tree-vect-loop.cc (vect_create_epilog_for_reduction):
	Do not register accumulator if we failed to reduce it
	to a single vector.

	* gcc.dg/vect/pr107160.c: New testcase.

(cherry picked from commit 5cbaf84c191b9a3e3cb26545c808d208bdbf2ab5)